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<h1 id="4">4.启动流程</h1>
<h2 id="41">4.1.描述</h2>
<p><img alt="img" src="../res/figure/boot-flow.png" title="玄武OS启动流程" /></p>
<h3 id="_1">低级初始化阶段</h3>
<p>低级初始化阶段，程序只能访问寄存器内存区，不可以访问全局变量。</p>
<h5 id="arch_lowlevel_init"><code>arch_lowlevel_init()</code></h5>
<p>架构层的低级初始化，例如：ARM的异常、数据cache、指令cache、
浮点运算单元等的初始化。</p>
<h5 id="cpu_lowlevel_init"><code>cpu_lowlevel_init()</code></h5>
<p>CPU层的低级初始化。</p>
<h5 id="soc_lowlevel_init"><code>soc_lowlevel_init()</code></h5>
<p>SOC层的低级初始化。</p>
<h5 id="board_lowlevel_init"><code>board_lowlevel_init()</code></h5>
<p>电路板的低级初始化。通常系统的时钟、电源等在这一层进行进行初始化。</p>
<h3 id="_2">系统初始化阶段</h3>
<h5 id="xwos_init"><code>xwos_init()</code></h5>
<p>初始化玄武OS的数据段(.xwos.data)，并初始化最简单的内核内存分配器。</p>
<h5 id="arch_relocate"><code>arch_relocate()</code></h5>
<p>初始化用户的数据段(.data)，经过此函数之后，用户才可以访问全局变量。</p>
<h5 id="arch_init"><code>arch_init()</code></h5>
<p>架构层的初始化。</p>
<h5 id="cpu_init"><code>cpu_init()</code></h5>
<p>CPU层的初始化。</p>
<h5 id="soc_init"><code>soc_init()</code></h5>
<p>SOC层的初始化。通常中断控制器在这一层进行初始化。</p>
<h5 id="board_init"><code>board_init()</code></h5>
<p>电路板的初始化。通常外设驱动在此处进行初始化，例如：SDRAM、GPIO、DMA等。</p>
<h5 id="cxx_init"><code>cxx_init()</code></h5>
<p>C++运行环境的初始化，玄武OS支持混合C++开发。</p>
<h3 id="_3">用户程序入口</h3>
<h5 id="xwos_main"><code>xwos_main()</code></h5>
<p>用户程序主函数，用户可在此处建立线程，启动调度器等。</p>
<h2 id="42">4.2.附录</h2>
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